1. Field of the Invention
This invention relates to a process and apparatus for the electrochemical extraction of metals dissolved in a molten electrolyte. The invention further relates to an apparatus for determining the metal composition, oxidation state of the metal species and their transport properties in a melt such as oxide and other slags in metallurgical reactors and various types of molten glasses.
2. Description of the Related Art
The extraction of metals from ores can be accomplished by pyrometallurgical or electrochemical means. Smelting is the predominant method of pyrometallurgical refining. In smelting, the ore is heated with a reducing agent and a flux to a high temperature. The reducing agent typically combines with the oxygen in the ore, yielding a pure metal or alloy and solid, liquid or gaseous oxide byproducts. The flux reacts with the oxide byproducts and with the unreacted components of the ore to form a liquid slag at the smelting temperature. Slag also refines the metal by incorporating one or more impurities. The slag can be physically separated from the refined metal. Smelting processes are used to extract iron, nickel, copper, lead, etc. from their ores. More metal is refined by smelting than by any other refining technique.
Electrolysis is the most common form of electrochemical refining. In an electrolysis process, the ore is dissolved in an aqueous or non-aqueous solution or melted in an electrolytic furnace. Once dissolved or melted, the ore dissociates into ionic species, forming an electrolyte. The metallic components of the ore to be extracted become positively charged cations. The remaining components, typically oxygen, carbonate, sulfate, chloride or fluoride become negatively charged anions. To extract the metal from the ore, an electric potential is applied across two electrodes which are immersed in the electrolyte. The metallic ions are thereby attracted to the negatively charged cathode, where they combine with electrons and are deposited as metal. The oxygen, sulfate, carbonate, chloride or fluoride ions are driven to the positively charged anode and evolve as waste gases. Aluminum, calcium, magnesium, and beryllium are examples of metals refined by electrochemical processes. Whereas electrochemical processes are usually preferred compared to pyrometallurgical processes, for quick energy efficient extraction and refining of metals, material selection for the electrolyte and process apparatus prevents broader application. They are usually restricted to the extraction of metals whose ores form very stable compounds.
Pal et al. in U.S. Pat. No. 5,567,286 describes a method for the electrochemical recovery of metals from slag using a galvanic (current producing) cell in which no external electric potential is applied. The refining process is driven by the chemical-potential gradient between the oxygen concentration within the slag and a refining gas which is separated from the slag by a solid electrolyte; however, because the chemical-potential gradient is fixed by the refining gas, the cell is not well suited for extracting on an industrial scale the desired metal from a melt that contains different metals.
Sammells et al. describe a cell for the formation of lithium metal and oxygen from molten metal salts containing lithium oxide, in which a cathode is immersed in a molten- salt electrolyte separated from an anode by a solid electrolyte. Sammells et al. rely upon high lithium cation mobility in order to drive the electrolysis reaction. The process as described by Sammells et al. requires suitable alkali ion conducting molten salt electrolyte having high alkali ion conductivity and thereby discourages applications involving transition metal and other cations with mobilities less than that of lithium. Also, the solid electrolyte used by Sammells et al is unsuitable for operation at higher temperatures because the solid zirconia-based electrolyte becomes partially electronic at high temperatures and short-circuits the cell, thereby reducing the efficiency of the cell.
Driven by the ever-rising demand for metals and the increasing scarcity of available mineral resources, there exists a need for an energy efficient, environmentally benign process for the refining of ores. Conventional electrolysis processes fail to meet these needs in that (1) the process is slowed by charge build up or polarization at the electrodes, (2) the electrolysis cell can get electrically shorted because the cathode and anode are both in the cell, (3) when refining metals with multiple or variable valencies parasitic reactions may occur at the anode and decrease the efficiency of the cell, and (4) product formation at the anode increases cell resistance.
Thus, there remains a need for a process and apparatus which will allow metals including non-reactive metals and metals with variable valencies, i.e., metals having more than one oxidation state, to be extracted from their respective ores via an electrolytic process that is environmentally sound and economically viable.
Sensors for the determination of oxygen concentration have been used extensively in the steelmaking process for better control of deoxidation, continuous casting, and ingot-making processes (see, Iwase et al. "Electronically Driven Transport of Oxygen from Liquid Iron to CO+CO.sub.2 Gas Mixtures Through Stabilized Zirconia" Metallurgical Transactions B 12B:517 (Sept. 1981)). Potentiometric sensors based on open-circuit techniques utilizing a metal/metal oxide reference are used extensively in the steel industry. These open-circuit measurements can give steelmakers an accurate evaluation of the oxygen activity and even the FeO.sub.x activity within a slag. However, potentiometric sensors can neither determine the actual concentration of FeO.sub.x nor provide information concerning the kinetics associated with diffusion within the slag. Also, slags with drastically different FeO.sub.x concentrations may have identical oxygen potentials, depending upon the structure and properties of the rest of the slag. Meanwhile the diffusion of FeO.sub.x species within the slag will be strongly dependent upon the intrinsic slag structure, basicity, and viscosity--none of which are directly measured in any way by the potentiostatic method. These variables are important because it is often the kinetics and not the thermodynamics which are important in controlling the slag/metal reactions of interest to steelmakers.
In addition to the information of oxygen activity provided by conventional oxygen sensors, there remains a need for rapid and accurate determination of the actual concentration and transport properties of metallic species in the slag. The importance of such information to the metals processing industry cannot be overestimated. Chemical analyses for FeO.sub.x in situ would allow steelmakers to control the slag chemistry by adding suitable fluxes thereby lowering the inclusion content of the steel, and information on the transport properties would allow the steelmakers to enhance the kinetics of the steelmaking process.
It is an object of the present invention to provide a method and apparatus for the extraction of metal from a metal-containing electrolyte which overcomes the deficiencies of the prior art.
It is a further object of the present invention to provide a method and apparatus for the extraction of metal from a metal-containing electrolyte which electronically separates the anode and the cathode.
It is another object of the present invention to provide a method and apparatus for the extraction of metal from a metal-containing electrolyte which is highly efficient, versatile, suitable for industrial scale processing and which may be used in the processing of a wide range of metals.
It is yet another object of the present invention to provide a method and apparatus for the determination of metallic species composition and transport properties in a metal-containing electrolyte.
These and other objectives of the present invention are achieved by practice of the present invention.